1. Field of the Invention
The present invention relates to an injection molding machine, and more particularly to a mold protection device for preventing molds form being damaged by a molded product or other foreign matter nipped by the molds in a mold closing process of an injection molding operation.
2. Description of the Related Art
In the mold closing process, if a molded product or other foreign matter is nipped by the molds, the molds may be damaged by the nipped foreign matter. To prevent such event, there is known a mold protecting method in which a mold protection region is set in a movable range of a movable mold before a contact position of the molds to reduce an output of a driving source of a mold clamping mechanism in the mold protecting region so that the movable mold is driven by a low torque and at a low pressure, and the motion of the movable mold is stopped if the foreign matter is nipped between the molds.
Also, for an injection molding machine in which a servomotor is used as a drive source of the mold clamping mechanism for clamping the molds, a mold protection method is known from Japanese Patent Publication No. 4-368832 in which a disturbance torque exerted on the servomotor is estimated by a disturbance estimating observer provided in a velocity loop of a servo control system in a mold closing process and if the estimated disturbance torque exceeds a predetermined value, an alarm is issued to stop driving of the servomotor to protect the molds.
In the above method of protecting molds by issuing an alarm when the disturbance torque estimated by the disturbance torque estimating observer exceeds the reference value, since a different mold set has a different allowable load and a different friction torque in motion, the reference value for discriminating an abnormal load is different for each mold set. Therefore, it is necessary to set a different reference value every time when a mold set is changed to another mold set in the mold clamping mechanism. If a reference value larger than the optimum value is set, a load greater than the allowable value may be exerted on the mold to be damaged when the molds nip the foreign matter.
If a reference value smaller than the allowable maximum value, since a normal disturbance load varying with fluctuation of friction force of the mold clamping mechanism in a mold closing motion and temperature of the molds, the normal disturbance torque quite lower than the allowable value is detected as a abnormal load to issue an alarm to stop the mold closing motion, to lower efficiency of operation.
In view of the above, it is necessary to set an optimum reference value for accurate discrimination of an abnormal load. However, it requires an operator to have adequate knowledge and experience on molds and injection molding operation since the optimum reference value varies depending on structure and temperature of molds, and environment of an injection molding machine. Even for the operator having adequate knowledge and experience on the injection molding operation it is very difficult to determine an optimum reference value.
An object of the present invention is to provide a mold protection device for an injection molding machine capable of automatically setting a reference value for detecting an abnormal load exerted on a mold in a mold closing process based on an average value of the estimated disturbance torques without need of manual input operation by an operator.
The mold protection device of the present invention comprises a disturbance estimating observer for estimating a disturbance torque exerted on a servomotor for driving a mold clamping mechanism at least within a mold protection region in every mold closing process, and a controller for defining an upper limit of the disturbance torque based on an average of disturbance torques estimated by the disturbance estimating observer in a plurality of mold closing processes, and for issuing an abnormal signal when a disturbance torque estimated by the disturbance estimating observer in a present mold closing process exceeds the upper limit in the mold protection region.
The disturbance estimating observer may estimate the disturbance torque at every predetermined period, and the controller may define the upper limit of the disturbance torque at each predetermined period based on an average of disturbance torques estimated by said disturbance estimating observer at every predetermined period in a plurality of mold closing processes, and issue an abnormal signal when a disturbance torque estimated by the disturbance estimating observer in a mold closing process exceeds the upper limit at each predetermined period.
The controller may define the upper limit based on an average of disturbance torques estimated by the disturbance estimating observer in a latest series of mold closing processes.
The disturbance torque estimated in a mold closing process in which an abnormal signal is issued is excluded in calculation of the average value.
The mold protection device of the present invention may further comprise a manual data input device for inputting a set value for defining the upper limit value in combination with the average value, and a display device for graphically displaying the estimated disturbance torque in the present mold closing process, the average value and the upper limit vale in relation to time or a position of a movable mold. The display device may graphically display a deviation between the average value and the disturbance torque estimated in a present mold closing process in relation to time or a position of a movable mold.
The estimated disturbance torques in a predetermined number of first molding cycles of an automatic injection molding operation may be excluded in the calculation of the average value since motion of the movable mold is not stable in these number of first molding cycles. The predetermined number of first molding cycles can be manually set through the manual data input device.